(1) Field of the Invention
The invention relates to the fabrication of integrated circuit devices, and more particularly, to a method to prevent corrosion of copper.
(2) Description of the Prior Art
In the creation of semiconductor devices, device elements are interconnected with metal traces, as are completed devices, for the performance of complex processing functions. Conventionally, aluminum has been used for the creation of interconnect traces. With increased device density and continued demands for device performance improvements, a substitute for aluminum has been explored and is at this time being used by using copper for the creation of interconnect traces. Other materials that are useful for the creation of conductive interconnect traces are tungsten, titanium, polysilicon, polycide or alloys of these metals.
Copper is increasingly used for interconnect traces due to its relatively low cost and low resistivity. Copper however has a large diffusion coefficient into silicon dioxide and silicon. Copper from an interconnect may diffuse into surrounding dielectric such as a silicon dioxide layer, causing the dielectric to become conductive and decreasing the dielectric strength of for instance the silicon dioxide layer. Copper interconnects are therefore typically encapsulated by at least one diffusion barrier to prevent diffusion of the copper into the silicon dioxide layer. Silicon nitride is a diffusion barrier to copper, but the prior art teaches that the interconnects should not lie on a silicon nitride layer because it has a high dielectric constant compared with silicon dioxide. The high dielectric constant causes an undesired increase in capacitance between the interconnect and the substrate. Also, copper has low adhesive strength to various insulating layers, and presents difficulties inherent in masking and etching a blanket layer of copper into intricate circuit structures.
In practical applications of creating conductive interconnect traces, the metal layers of Integrated Circuit (IC) devices are prone to be corroded and oxidized by various chemicals that are used during the processing of the IC devices. For example, copper is corroded if it is exposed to a strong alkaline solution. The invention provides a structure of preventing this corrosion of copper, using the Pourbaix diagram of copper and therewith applying a method of impressed-current cathodic protection. A process will be highlighted that further explains the exposure that is suffered by copper that is used as part of the creation of an interconnect bump. The invention provides a structure of impressed-current cathodic protection to prevent corrosion of the metal that is used for the creation of for instance interconnect traces or interconnect studs or bumps.
U.S. Pat. No. 6,274,504 B2 (Sanderfer et al.) shows a process to minimize corroding during post metal solvent clean.
U.S. Pat. No. 6,209,551 B1 (Yang et al.) is a related strip process.
U.S. Pat. No. 5,336,371 (Chung et al.) shows a photo-resist stripper tool that reduces corrosion.
The Pourbaix diagrams that are discussed as part of the invention are further detailed in the publication “Atlas of Electromechanical Equilibria in Aqueous Solution”, published 1974, ISBN 0-915567-98-9.